When sailing with a passenger, he/she typically holds onto either the righting line or another line I keep secured to the mast. While this is helpful for maintaining some level of balance most of the time, it sometimes backfires in rough waters when the leeward hull dips underwater and the Wave slows down really quickly. This sometimes causes the passenger to fall forward, disrupting the weight distribution and sometimes leading to a pitchpole event. Any tips for creating something to prevent this?

When sailing with a passenger, he/she typically holds onto either the righting line or another line I keep secured to the mast. While this is helpful for maintaining some level of balance most of the time, it sometimes backfires in rough waters when the leeward hull dips underwater and the Wave slows down really quickly. This sometimes causes the passenger to fall forward, disrupting the weight distribution and sometimes leading to a pitchpole event. Any tips for creating something to prevent this?

sure - that's the objective. It's easy for whomever is at the helm to deal with a sudden deceleration, as he/she is holding onto the mainsheet line, which is secured to the rear of the boat. The passenger doesn't have the same luxury, so dealing with sudden deceleration from 15 knots to 3 knots is not easy when the only line they're holding on to is secured to the mast. I suppose securing a separate line to somewhere on the tramp near the rear of the boat is worth a try.

Why don't you just have them hold onto the shroud (side stay)? It shouldn't move much.

If you want to install an extra grab-line, you can make use of the threaded inserts on the outsides of the hull, about half-way back. Pop the plastic plugs out and find an eye of the right spacing. Tie a single line to that, or tie a chicken-line from it back to a pintle.